Department of Geology and Environmental Studies (Geology home page jmu.edu/geology/)

James Madison University, Harrisonburg, VA 22807
Spring, 1999

The Cyclical Opening and Closing of Ocean Basins

No rock is accidental. No idea in geology is more
profound than this; it runs from the center to the whole of geology and influences every subdiscipline
of the field. Genuine understanding of the science of geology begins with one's ability to
understand and explain why no rock is accidental.

Tectonics is concerned with deformation in the earth
and the forces which produce deformation. Plate tectonics is the theory that the earth's lithosphere
(outer rigid shell) is composed of several dozen "plates", or pieces, which float on a ductile mantle,
like slabs of ice on a pond. In plate tectonic theory earth history, at its simplest, is one of
plates rifting into pieces diverging apart and new ocean basins being born, followed by motion
reversal, convergence back together, plate collision, and mountain building. This cycle of opening
and closing ocean basins is the Wilson Cycle.
Plate tectonics is one of the great unifying theories in geology. Virtually every part of the
earth's crust, and every kind of rock and every kind of geology can be related to the plate tectonic
conditions which existed at the time they formed. Nothing in geology makes sense except in terms of
plate tectonic theory.
One of the most important messages of modern understanding of plate tectonics and the Wilson
cycle is that beginning with a parent igneous rock of mafic/ultramafic composition all the other
rocks now on the earth can be generated. The most important message of the plate tectonic rock
cycle is that each and every rock forms only under a specific set of tectonic conditions.

Most geologic activity occurs at the three kinds of plate boundaries: (1) divergent boundaries
where plates are moving apart and new crust is being created,

(2) The Tectonic Rock Cycle, a more theoretically abstact model of how rocks and the earth evolve.The following Wilson Cycle model follows the series of cross sections constituting the Wilson cycle. It begins with a hypothetical geologically (tectonically) quiet continent. The model is divided into nine stages, but the stages are arbitrary and do not exist naturally. The earth is an ongoing series of processes so it is much more important to understand the processes, how they are related, and how one process leads naturally to the next process.
Also note that this Wilson Cycle is a simple, ideal model. The earth has many continents, which migrate across its spherical surface in very complex ways. Just about any scenario you can think of, and any exception you can imagine is quite possible - and has probably happened during some point in the earth's history.
Go to Stage APDF version of illustrations for all stagesThe Plate Tectonic Rock Cycle